In drilling machines for rock drilling having a drill string and a drill bit at its distal end, the drill string adjoins to a drilling machine over an adapter, having its axial contact with the drilling machine against a drill bushing. In a previously known percussion drilling machine with rotation means, the contact between the adapter and the drill bushing is typically achieved through contact of the latter against axial ends of the rotational splines of the adapter.
Against the end of the drill bushing which is turned away from the drill string, acts the so called damping piston, which functions so as to transfer the feed force against the rock from the machine housing to the drill bushing and further over the adapter, over the drill string to the drill bit for contact thereof against the rock. According to previously known art, the damping piston is preloaded through the hydraulic/pneumatic spring being comprised of hydraulic fluid in a chamber, which often has a connection to a hydraulic/pneumatic accumulator.
If the shockwave produced by the percussion piston through the drill string is not adapted to the impedance of the rock, the result will be reflexes back through the drill string. A reflex often includes a tensile stress portion as well as a compressive stress portion. If the rock is hard in relation to the shockwave force, the result will in general be compressive reflexes, having amplitudes twice the size of the incoming shockwave. If, on the contrary, the rock is soft or if the drill bit has inferior contact with the rock, tensile waves having essentially the same amplitude as the incoming shockwave will be reflected.
The compressive reflexes force the drill bushing and the damping pistons in the direction from the drill string, whereby hydraulic oil is charged into the accumulator. The pressure therein then presses back the damping piston and the bushing to the original position against a mechanical stop in the machine housing. Resilience in the accumulator thus provides an elastic function, which protects the drilling machine against high stresses and vibrations. This increases the working life of the drilling machine and makes is possible to transfer greater effects.
Tensile reflexes can not be handled directly by damping systems of the older type. A sudden cavity in the rock can thus result in that the drill string for a short while separates from the drilling machine. This results in ineffective drilling.
In a so called floating damper, there exists a constant hydraulic fluid flow through the damping chamber. In this arrangement the pressure is temporarily reduced in the damper if the damping piston passes a floating position in the direction of the drill string. The feed force can then more effectively reset the floating position by displacement of the machine housing in the direction of the drill string such that a better contact with the drill string is obtained. A problem with this kind of arrangement is the increased energy consumption which is caused by the need of a constant flow of hydraulic liquid.
With a so called single damper, the damping piston is actuated in the drilling direction of a volume pressurized to the pressure of the percussion device against a fixed end stop in the machine housing. In particular in case of low feeding force, contact between the damping piston and the shank adapter is lost already before the strike occurs. Reflected shock wave energy from the rock result in that the damping piston receives a speed backwardly in the machine housing, where after, following a shift of the damping piston forwards, it strikes with essentially the same speed against the shank adapter and its' end stop. Also during this movement, there is a lack of transfer of feeding force to the drill string and thereby rotational resistance.
In a variant of the single damper, the damping piston is balanced around a floating position, which is determined by a constant oil flow. This solution allows that the damping piston is capable to follow the shank adapter for enhanced rock contact at the cost of an increased consumption of effect.
In a so called double damper, the damping piston has two pressurized areas. One of these areas exists in a confined volume which acts as a throttle damper, and which transforms reflected energy of a certain magnitude into heat in the hydraulic oil when it passes through a throttling slot. Also double dampers can be made floating around a floating position.
As background art can be mentioned EP 0856 657 A1 and WO2004060617 A1.
Altogether, damping systems according to the background art have resulted in that fast reaction and good energy absorption has been able to obtain at the expense of a certain energy consumption. More energy saving damping devices have, however, resulted in slow systems with relatively long periods without contact between a damping piston and the adapter, resulting in that during considerable periods, no feeding force has been transferred to the drilling steel, whereby no tightening of joints occurs in the drill string and ineffective drilling will be the result.